This invention relates to a counterflow heat exchanger having two fixed tube plates and a thermal exchange zone comprising substantially straight tubes, and in particular to a heat exchanger suitable for high pressure and temperature service, to be employed in either conventional or nuclear power stations, as well as in other industrial plants.
As is known, several industrial plants make use of counterflow heat exchangers which are of considerable size, and owing to the severe operating conditions encountered, expected to provide the highest and most comprehensive degree of reliability, both to avoid halting the plants, with obviously heavy consequences of an economical nature, and for inherent safety reasons. Typical examples are the steam generators using sodium as the primary fluid, which are installed at nuclear power stations of the LMFBR type.
Heretofore, such heat exchangers comprised, in the majority of cases, a pair of oppositely located tube plates, arranged to face each other at a distance apart, which are interconnected by a nest of tubes welded to the plates themselves in a manner that will be explained hereinafter, for the passage of the secondary fluid; also provided is a shroud or outer casing which connects the tube plates to each other such as to enclose the tube nest and confine the primary fluid passage zone.
The structure of such heat exchangers, as well as that of other known designs, has first of all the serious disadvantge--which affects in particular the cited steam generators using sodium as the primary fluid--of a disuniform primary fluid flow at the thermal exchange zone, which flow, at the usually circular center portion of the stream section, has a higher velocity than at the periphery thereof; this results in a non-uniform distribution of the wall temperature in the various tubes, with attendant negative consequences, particularly of a mechanical and structural nature, as the expert will readily recognize.
Also, the inlet and outlet flows of the primary fluid are imperfectly uniform, as dictated by the provision of conventional annular headers, usually arranged externally to the tube nest.
Furthermore, it is known that in high reliability heat exchangers, the best procedure currently adopted for welding the tubes to the tube plates is one selected from the IBW (Internal Bore Welding) techniques and enables the tubes to be butt welded to spigot members, purposely formed on the plates and bored to a diameter which is substantially equal to the inside diameter of the tube; more specifically, this type of weldment, which is known per se, provides for the end of the tube to be welded to fit inside a seat formed on the spigot, as prearranged on the tube plate, thereafter access is gained with a welding torch to the tube inside, at the joint area, to carry out the welding, usually without deposition of any weld material.
This type of weldment, especially in view of the severe operating conditions anticipated for the cited exchanger designs, must then be individually checked, in general by X-ray or ultrasonic inspection, to ascertain its reliability.
Now, as mentioned, in most heat exchangers of conventional design, the tube plates are arranged to face each other from a distance apart, thereby in order to allow the individual tubes which make up the nest to be installed in conformity with accepted manufacturing and inspection practices, it becomes necessary to provide one plate with bores having substantially the same size as the tube outside diameter, whereas the other plate can be prefabricated with spigots having seats adapted for convenient application of the aforementioned welding procedure.
A serious problem encountered with conventional heat exchangers of this type is that the requirement of providing one plate with bores having the same size as or a size slightly larger than the tube outside diameter makes it necessary to insert the tube end for a few millimeters inside the bore, purposely made oversize to accommodate the tube, thereby, when this tube end is welded to the tube plate, the weldment area acquires a substantially truncated cone flare out rather than being rectilinear.
The presence of this flare-out at the weldment area has first of all the disadvantage of being liable to undergo deflection and shear actions, which are technically undesirable in this type of joint, and moreover this type of weldment is difficult to X-ray, such that considerable problems are encountered during the inspection step; trouble may also arise from the fluid dynamics characteristics which result therefrom.
A further serious drawback of the heat exchangers of the type just described, as well as of other conventional such designs, resides in that for obvious reasons of construction and inspection at least part of the outer casing or shroud must be attached to the tube plates after welding the tube nest to the plates, thereby considerable difficulty is experienced when it comes to X-raying the shroud weldments, while it is impossible to reweld the weldments because no access can be had to the inside. It should be further added to the foregoing that when the shroud is connected to the tube plates after the installation of the tube nest, any heat treatment of the welded areas of the shroud becomes extremely difficult to carry out.
Provision may be mde in the heat exchangers of the type described above for the presence of an expansion joint in the shroud effective to accommodate thermal expansion differentials between the tube nest and shroud; while in the latter case there still exists the possibility of the whole tube nest expanding, any expansion differentials, as originating from various causes, between the tubes are nevertheless prevented, an example of such causes being a different flow distribution from one tube to another. These expansion differentials unavoidably generate tensions that concentrate at the weldment areas of the tubes to the tube plates, which brings about obvious risks and trouble especially with joints of conventional type, where as mentioned deflection and shear stresses are induced.
Still another drawback of almost all the known types, and one which is more markedly evident when high pressures are involved, is that the forged stock used for forming the tube plates have a large mass, which adds complications of mechanical, thermal, and metallurgical natures. Also considerable is the difficulty of assembling the tube nest.